In radiography, particularly in medical radiography, use is normally made of radiographic systems comprising an intensifying screen and a radiographic film containing at least one radiation sensitive silver halide emulsion layer. The use of intensifying screens makes it possible to reduce the quantity of X-rays required to obtain a radiograph and consequently to reduce the quantity of X-rays absorbed by the patient.
The photosensitive emulsion in the radiographic film is spectrally sensitized with a dye having a peak absorption that matches as closely as possible the maximum emission band of the intensifying screen. The radiographic film is comprised of a transparent (often blue tinted) film support coated on one of its faces and, preferably on both faces, with at least one layer of silver halide emulsion containing radiation sensitive silver halide grains.
Since the early 1980's it has been recognized that significant performance advantages can be realized by employing spectrally sensitized tabular grain emulsions in radiographic film. Spectral sensitization is achieved by adsorbing a spectral sensitizing dye having an absorption peak matched as closely as possible to the peak emission of the intensifying screen. The absorption peak can lie in any convenient region of the spectrum--including the blue, green or red region of the spectrum. For example, when an intensifying screen is employed that emits in the blue spectral region between 360 and 500 nm, a spectral sensitizing dye is employed having a maximum absorption in this same spectral range.
Daubendiek et al U.S. Pat. No. 4,639,411 describes a radiographic element which comprises an emulsion consisting of tabular silver halide grains capable of forming a latent image when it is exposed to light. This emulsion preferably consists of tabular grains with a thickness of less than 0.5 .mu.m and an aspect ratio higher than 5:1.
Tabular grain emulsions achieve maximum sensitivity at a high level of sensitizing dye because of the high ratio of grain surface area to grain volume. A problem of residual dye stain has therefore emerged with the use of spectrally sensitized tabular grain emulsions in radiographic film. The dye stain is observed as a residual coloring in the areas which are unexposed or only slightly exposed. The problem is particularly objectionable when the tabular grains can a blue spectral sensitizing dye. As is understood in the art, blue spectral sensitizing dyes selectively absorb blue light and, hence, are yellow in color. The yellow color imparted by residual, unrermoved blue spectral sensitizing dye imparts a warm image tone to the film. That is, instead having a neutral color the black-and-white imaged film is shifted toward a brown appearance. In medical radiography user preference is for cold (blue black) image tones. The problem of dye stain is exacerbated by rapid processing. In rapid processing imagewise exposed dry radiographic film is supplied to a processor and emerges from the processor dry bearing a developed image in less than 1 minute. The short processing interval limits the time available for spectral sensitizing dye transfer from the film to the processing solution.
In order to avoid dye stain, a common approach is to limit the choice of spectral sensitizing dye. This invention makes it possible to use spectral sensitizing dyes which, if they were used alone in high quantities, would have caused a residual coloring in the areas of the image of low density. According to the invention, this coloring is avoided by associating therewith an optical brightener having special characteristics. One advantage of the invention is that it affords a greater degree of freedom in the choice of spectral sensitizing dyes. In addition, surprisingly, the sensitometric properties of the radiographic image are not substantially impaired by the presence of the optical brightener according to the invention.
The optical brighteners which have been known for a long time are substances which absorb ultraviolet light in the region of the spectrum lying between 300 and 400 nm and which are fluorescent in the blue region of the visible spectrum. They are used for improving the whiteness of photographic papers and in photographic films. Research Disclosure of July 1983, No 23136, describes inter alia the use of optical brighteners in radiographic systems. According to this article, the presence of optical brighteners reduces the yellow appearance of the supporting polymer when a source of ultraviolet light is used. The optical brightener is incorporated in a layer placed on the support, or on the support by soaking the support in a solution containing the optical brightener, for example a developing or fixing solution or a stabilizing bath. This article does not deal with the problem of the residual coloring due to the spectral sensitizing dye.
McFall et al U.S. Pat. No. 2,933,390 describes a supersensitization method of silver halide emulsions which combines the use of diamino stilbene optical brighteners with dicarbocyanine dyes. Supersensitization which is a technique well described in the literature, consists in increasing the spectral sensitivity of a dye by the addition of another substance showing a strong adsorption in the near UV, and having an electron system similar to the dye with which it is associated. McFall associates specific dicarbocyanine dyes with the aminostilbene. The supersensitization would not be observed if dyes other than those of McFall were used e.g. carbocyanine dyes or, as in the present invention mono methine cyanine dyes. Also, the problem raised and solved by the present invention is very remote from the problem addressed in McFall U.S. Pat. No. 2,933,390, and certainly not described, even inherently, in this reference.
Kuse U.S. Pat. No. 4,232,112 describes a process for developing a color photographic film in which the development solution or the bleaching/fixing solution comprises a bleaching agent which is a derivative of 4,4'-diamino-stilbene disulfonic acid and a second compound derived from benzene sulfonic or naphthalene sulfonic acid. This process makes it possible to avoid on the one hand the residual coloring due to oxidation of the color developer in the bleaching/fixing solution and on the other hand the residual coloring which is due to the accumulation of various substances on the edges of the film.
Ishikawa U.S. Pat. No. 4,587,195 describes a method for eliminating the residual coloring in a color image. For this purpose a development solution is used comprising a combination of optical brighteners of the triazylstilbene type with slightly shifted absorption peaks.
Hoyen U.S. Pat. No. 5,238,793 describes a method of processing black and white films, particularly microfilms, comprising a layer of silver halide emulsion spectrally sensitized with a cyanine dye. When the rate of replenishment of the processing solution is low or when the level of sensitizing dye is high, the spectral sensitizing dye forms in the processing solutions solid particles which adhere to the surface of the film and form specks or blotches on the final image. The problem is solved when the exposed film is processed in the presence of a stilbene which enables the spectral sensitizing dye to dissolve. Neither radiographic film, tabular grain emulsions, nor dye stain is addressed by Hoyen.